This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Objective: To understand human presbyopia by studying the dynamics of accommodation and presbyopia in nonhuman primates. Nonhuman primates are the preferred model for human presbyopia, the age-related loss of the ability to focus. The equatorial diameter of the monkey lens does not change with age;thus, as in humans, presbyopia can not be attributed to an age-related increase in lens diameter. Further, we determined the real-time dynamics of the intraocular structures involved in accommodation during midbrain stimulation. These studies generated new information about the mechanisms of accommodation. There is an age-related change in the geometry of the accommodative apparatus;with increased age the distance between the ciliary muscle and the posterior zonule (the elastic fibers extend from the zonular plexus anteriorly to the vitreous membrane near the posterior end of the muscle) increases. The posterior zonule and its posterior insertion point may moderate ciliary muscle contraction and accommodative amplitude and may dampen the ciliary muscle accommodative response in the older eye. Also, the capsule facilitates forward accommodative ciliary body movement. Lens substance extraction procedures that leave the capsule intact, similar to those used clinically in humans, do not affect the accommodative system movements, an important finding for the development of accommodating intraocular lenses (IOLs). Whether the age-related changes in geometry and posterior zonule function will allow the older accommodative apparatus to still mold an accommodating IOL would depend on the characteristics of the IOL and how much force the capsule can exert. Unless the older posterior zonule can be severed, accommodating IOLs may be more effective in restoring accommodation in the presbyopic eye if they rely on centripetal muscle and capsular movements rather than on forward muscle movement. This research used WNPRC Research Services.